1. Field of the Invention
The present invention relates to testers, especially those testers which include at least two input terminals, a galvanometer capable of visual indication of measured values, and a network which is located between the input terminals and the galvanometer and capable of converting signals generated at the input terminals to the level suitable for the operation of the galvanometer.
2. Description of the Prior Art
The circuit tester is in wide use for a variety of measurements on electric circuits, e.g., for voltage measurements in a wide range from several hundred mV to ca. 1000 V. Given such uses, it is inevitable that a tester, set for a lower voltage range, may be used, by mistake, to measure voltages higher than the maximum voltage setting on the tester. Such mistaken operations often result in damage to meters, circuit elements (such as resistors), or patterns, within the tester.
As a protective means for circuits to guard against such mistaken operations, some conventional testers have fuses equipped in their input circuits. The use of fuses, however, results in disadvantages in that measurements of electric currents result in errors since the fuse possesses a finite resistance and the measurement accuracy is lowered since the fuse resistance is temperature dependent. When an excess current is caused to flow, it takes the fuse some period of time to burn out, sometimes resulting in damage to the internal circuits. The fuse needs to be replaced after burn-out. Thus, the use of a fuse has specific disadvantages from the standpoint of maintenance of the tester.
Some testers have been designed to allow their relays to detect overcurrent and to cut off their input circuits for measurement. Japan Pat. No. 53-38481/1978 describes an example tester of this sort which uses a latching relay. This latching relay has a temporary stable state and a semipermanent stable state, the transition from the former to the latter state being effected by a flow of current over the working range through the coil, the reverse transition by external mechanical force being exerted via manual operation, etc. In general, relays are suitable for protecting tester circuits since they have low contact resistance with low temperature dependence and are capable of rapid reaction. In addition, the use of relays permits the protective operation to be executed at the same multiplying factor among all the measurement ranges, so that they may protect not only the current measuring circuits but also the voltage measuring circuits as well. The tester using the latching relay may cut off its input circuit in response to an excessive current by shifting into the semipermanent state. However, this tester has operational disadvantages in that manual operation is required to reset the relay from the semipermanent to the temporary stable state, and in that, before the cause for the input of the excessive current is removed, the tester may reset its relay resulting in a closed input circuit having the same excessive input current to its measuring circuit. It has an additional structural disadvantage in that, since the temporary stable state is a so-called "unstable stable-state", where the relay has its set of springs under some tension, the tester, when in the temporary stable state, may have its input circuit cut off in response to an accidental external force such as vibration, shock, etc. Such a structure makes the relay so large that it is difficult to build such a relay in a small-sized tester.